Electrical overstress, or EOS, refers to the destruction of a circuit due to excessive voltage, current, and/or power.
EOS is usually caused by improper application of excitation to a circuit, whether the circuit still being tested in the manufacturing line or it is already in the field. Simple socketting violations such as circuit misorientation and shifting can cause EOS damage, especially if the voltages intended for the power supply pins will be applied to stress-sensitive or power-limited pins. Improper excitation settings or voltage spikes in the excitation source are also common causes of EOS damage.
EOS damage is not always obvious. Some EOS events leave no apparent physical manifestation at all. Such EOS events can still render the affected component non-functional, even if no physical anomalies are observable. Weak EOS events may also occur, simply shifting the parametric performance of the affected component, but nonetheless affecting the over-all performance of the circuit.
Electrostatic Discharge (ESD) and latch-up and are two special cases of EOS.
An electrostatic discharge (ESD) is a sudden flow of electric current through a material that is normally an insulator. A large potential difference across the insulator may generate a strong electric field, converting the material's atoms into ions that conduct a current.
Electro-static discharge (ESD) may also be defined as a single-event, rapid transfer of electro-static charge between two objects, usually resulting when two objects at different potentials come into direct contact with each other.
ESD is a major cause of device failure in the semiconductor industry because integrated circuits are made from insulating materials such as silicon, which can break down if exposed to high voltages. Manufacturers and users of integrated circuits must take precautions to avoid and/or account for ESD.
A built-up electrostatic charge may be quickly discharged when the charged device comes into contact with another device, for example, an integrated circuit, especially when portions of the integrated circuit are connected to power supplies including ground. The electrostatic discharge may cause damage to the integrated circuit by causing dielectric breakdown of oxides and other thin films within the integrated circuit.
Dielectric breakdown refers to the destruction of a dielectric layer, usually as a result of excessive potential difference or voltage across the dielectric layer. Dielectric breakdown may be manifested as a short or leakage at the point of breakdown.
SiO2 breakdown is believed to be due to charge injection, and may be broken down into two stages. During the first stage, current starts to flow through the oxide as a result of the voltage applied across it. High field/high current regions are then formed as charges are trapped in the oxide. Eventually, these abnormal regions reach the second stage, a critical point wherein the oxide heats up and allows a greater current flow. This results in an electrical and thermal runaway that quickly leads to the physical destruction of the oxide.
Damage to an integrated circuit may also result from higher than normal levels of conduction through relatively small areas of the integrated circuit arising from reverse breakdown of p-n junctions in the integrated circuit.
Latch-up pertains to a failure mechanism wherein a parasitic thyristor (for example, a parasitic silicon controlled rectifier, or SCR) is inadvertently created within a circuit, causing a high amount of current to continuously flow through the circuit once the current is accidentally triggered or turned on. Depending on the circuits involved, the amount of current flow produced by this mechanism can be large enough to result in permanent destruction of the device due to electrical overstress (EOS).
An SCR is a three-terminal, four-layered p-n-p-n device that basically includes a PNP transistor and an NPN transistor. An SCR is off during its normal state but will conduct current in one direction (from anode to cathode) once triggered at its gate, and will do so continuously as long as the current through it stays above a holding level. Triggering the emitter of the PNP transistor into conduction injects current into the base of the NPN transistor. This drives the PNP transistor into conduction, which forward biases the emitter-base junction of the NPN transistor further, causing the NPN transistor to feed more current into the base of the PNP transistor. Thus, the NPN transistor and the PNP transistor feed each other with currents that keep both of them saturated.
Events that can trigger parasitic thyristors into a latch-up condition include excessive supply voltages, voltages at I/O pins that exceed the supply rails by more than a diode drop, improper sequencing of multiple power supplies, and various spikes and transients. Once triggered into conduction, the amount of current flow that results may depend on current limiting factors along the current path. In cases where the current is not sufficiently limited, EOS damage such as metal bum-out can occur.
FIG. 1 illustrates a conventional SCR, used to control ESD. As illustrated in FIG. 1, a conventional SCR may include a first diffusion layer 2, for example of n-type, a second diffusion layer 4, for example, of p-type, a third diffusion layer 6, for example, of n-type, and a fourth diffusion layer 8, for example, of p-type. A conventional SCR may also include an n-well 10, a p-well 12, a first pad, for example, a VDD pad, and a second pad 14, for example, a VSS pad.
As shown in FIG. 1, a pnpn SCR is effectively connected between a protected line and each terminal of the power supply, VDD and VSS. The SCR essentially includes a first pnp bipolar transistor Q1 and the second npn bipolar transistor Q2. As shown in FIG. 1, an emitter (for example, the second diffusion layer 4) of the pnp bipolar transistor Q1 and the collector (for example, the first diffusion layer 2) of the npn bipolar transistor Q2 are connected to one of the terminals, for example, the first pad 13. The emitter (for example, the third diffusion layer 6) of the npn bipolar transistor Q2 and the collector (for example, the fourth diffusion layer 8) of the pnp bipolar transistor Q1 may be connected to the other terminal, for example, the second pad 14. As shown in FIG. 1, the base of the npn bipolar transistor Q2 may have the same diffusion (for example, the p-well 12) as the collector of the pnp bipolar transistor Q1 and the base of the pnp bipolar transistor Q1 may have the same diffusion (for example, the n-well 10) as the collector of the npn bipolar transistor Q2.
As set forth above, a conventional SCR is a normally off device, in a “blocking state”, in which negligible current flows therethrough. In its normally off state, a conventional SCR prevents a high impedance path between power supplies.
During an ESD event, for example, when the first pad 13 is at a high voltage and the second pad 14 is a ground, a circuit path for ESD current may be traced from VDD to VSS in separate paths, for example, a pnp path and an npn path, through the two bipolar transistors Q1, Q2 that form the SCR. Current is injected into the base of the npn bipolar transistor Q2, which causes a current flow in the base-emitter junction of the pnp bipolar transistor Q1. This current flow causes the pnp bipolar transistor Q1 to turn on causing further current to be injected into the base of the npn bipolar transistor Q2. This phenomenon is called a “positive-feedback condition”, which pushes the SCR into a regeneration mode from a low impedance discharge channel to safely shunt the ESD current.
FIG. 2 illustrates an example ESD characteristic curve of two conventional ESD protection devices, including a conventional SCR, for example, such as the conventional one described in the FIG. 1, illustrated as curve A, and a conventional pn diode, illustrated as curve B. As shown in FIG. 2, the ESD characteristic curve for a conventional SCR includes three distinct portions 1-3. In portion 1, under an ESD-stress condition, the voltage V is less than a terminal voltage VT, which results in a high-impedance condition, where the potential of the n-well 10 increases.
At the beginning of portion 2, where V=VT, a junction breakdown (or avalanche breakdown) occurs at the common collector-base junction. The avalanche-generated holes raise the p-well 12 potential and avalanche-generated electrons raise the n-well 10 potential, which turns on the npn and pnp bipolar transistors.
Which of the npn and pnp bipolar transistor turns on first depends on the values of resistors R1 and R2, and the current gains of the two bipolar transistors. Latch-up occurs when a voltage spike activates one of a pair of bipolar transistors, which combine a circuit with a large positive feedback. As a result, a large current can flow out through the collectors. As the SCR moves into the “on” state, an I/O pad voltage may be clamped to a safe holding voltage. This results in a drastic decrease in the voltage to a hold voltage voltage VH, as shown in portion 2 of curve A of FIG. 2.
In portion 3, if a current inflow continues or a voltage over VH is applied on the pad, this results in a low-impedance condition. Latch-up may be defined as the creation of a low-impedance path between power supply rails as a result of triggering. In this condition, excessive current flow is possible and the device may go into thermal breakdown. The device temperature may increase to such a level that thermal carrier generation is high enough to dominate the conduction process. Because the current flow in portion 3 is too high, localized thermal damage may occur in the semiconductor device. The uncontrolled current increase shown in portion 3 of curve A is a problem with conventional SCR devices.
Curve B illustrates the ESD characteristic curve of a conventional pn diode. As shown in portion 1 of curve B, current increases very slowly; as a result, a conventional pn diode is not suitable for instantaneously discharging a large current.